A wheel 1 constituting a vehicle wheel, and a rotor 2 constituting a disc brake being a braking device, are rotatably supported on a knuckle 3 constituting a suspension system, for example by the structure shown in FIG. 5. That is, an outer ring 6 being a stationary ring constituting a wheel bearing unit 5 is fixed to a portion of a circular supporting hole 4 formed on the knuckle 3, by a plurality of bolts 7. On the other hand, the wheel 1 and the rotor 2 are securely connected to a hub body 8 constituting the wheel bearing unit 5, by a plurality of studs 9 and nuts 10.
Double row outer raceways 11a and 11b are formed on the inner peripheral surface of the outer ring 6, and a fixing side flange 12 is formed on the outer peripheral surface. Such an outer ring 6 is secured to the knuckle 3 by connecting the fixed side flange 12 to the knuckle 3 by the bolts 7.
On the other hand, a rotation side flange 13 is formed on a part of the outer peripheral surface of the hub body 8, on a portion protruded beyond the outside end opening of the outer ring 6 (outside in the axial direction means the side towards the widthwise outside when assembled in the vehicle; the left side in the respective drawings, while conversely, the side towards the widthwise center when assembled in the vehicle is the inside in the axial direction; the right side in the respective drawings). The wheel 1 and the rotor 2 are firmly secured to one side face (the outside face in the illustrated example) of the rotation side flange 13 by the respective studs 9 and the nuts 10. Moreover, an inner ring raceway 14a is formed on the outer peripheral surface of a middle portion of the hub body 8, on a portion facing the outer ring raceway 11a being the axial outside raceway of the double row outer ring raceways 11a and 11b. Furthermore, an inner ring 17 constituting a hub 16 together with the hub body 8 is externally secured to a small diameter stepped portion 15 formed on the outer peripheral surface of the inside end of the hub body 8. An inner ring raceway 14b formed on the outer peripheral surface of the inner ring 17, faces the outer ring raceway 11b being the axial inside raceway of the double row outer ring raceways 11a and 11b. 
A plurality of balls 18 serving as rolling elements are rotatably provided respectively between the outer ring raceways 11a and 11b, and the inner ring raceways 14a and 14b, in a state of being respectively retained by cages 19. By means of this structure, a double row angular type ball bearing, being a back-to-back duplex bearing is constituted, and the hub 16 is rotatably supported inner side of the outer ring 6, while freely supporting radial load and thrust load. Seal rings 20a and 20b are respectively provided between the inner peripheral surfaces of the opposite ends of the outer ring 6 and the outer peripheral surface of the middle portion of the hub body 8, and the outer peripheral surface of the inside end of the inner ring 17. Therefore, an interior space 21 in which the respective balls 18 are provided, is closed off from the outside.
Furthermore, since the illustrated example is a wheel bearing unit 5 for supporting a driven wheel (rear wheels of front-engine rear-drive vehicle and rear-engine rear-drive vehicle, front wheels of front-engine front-drive vehicle, and all wheels of 4-wheel-drive vehicle), a spline hole 22 is formed in the center of the hub body 8. In use, a spline shaft 24 of a constant velocity joint 23 is inserted into the spline hole 22. The inner diameter side of the rotor 2 is connected to and supported on the outside face of the rotation side flange 13 provided on the outer peripheral surface of the hub body 8. When assembled in the vehicle, in a state where the spline shaft 24 of the constant velocity joint 23 is inserted into the spline hole 22, and the outside end face of a housing 25 constituting the constant velocity joint 23 comes up against the inside end face of the inner ring 17, a nut 26 is screwed onto a tip of the spline shaft 24, on a portion protruded beyond the outside end face of the hub body 8, and secured thereto. According to this structure, the inner ring 17 is kept from being displaced in the axial direction with respect to the hub body 8.
In the structure in the first example described above, the knuckle 3 and the outer ring 6 constituting the wheel bearing unit 5, which are constructed independently to each other, are securely connected by the plurality of bolts 7. On the other hand, in U.S. Pat. No. 6,485,188, Japanese Laid-open Patent Publication No. 2001-1709 and Japanese Laid-open Patent Publication No. 2002-46409, a structure is described where the knuckle and the outer ring of the wheel bearing unit are integrated, so as to omit the fixed side flange and the plurality of bolts for connecting the knuckle and the outer ring. FIG. 6 shows a structure described in U.S. Pat. No. 6,485,188.
In the case of the second example of a conventional structure shown in FIG. 6, an outer ring 6a having a convex portion 27 on the outer peripheral surface, is inserted into a knuckle 3a when casting the knuckle 3a, so that the outer ring 6a and the knuckle 3a are integrally constituted. Moreover, on the inner diameter side of the outer ring 6a, a hub 16a comprising a hub body 8a and a pair of inner rings 17a and 17b, is rotatably supported by a plurality of tapered rollers 28 being rolling elements.
Furthermore, in Japanese Laid-open Patent Publication No. 2001-1709 and Japanese Laid-open Patent Publication No. 2002-46409, although not shown, a structure is described where double row outer ring raceways are formed directly on the inner peripheral surface of a circular supporting hole which is formed in the knuckle, or alternatively an outer ring is secured to the knuckle by welding, so as to have the knuckle and the outer ring integrated.
According to such structures described in U.S. Pat. No. 6,485,188, Japanese Laid-open Patent Publication No. 2001-1709 and Japanese Laid-open Patent Publication No. 2002-46409, the total weight of the knuckle and the wheel bearing unit can be reduced, so that the running performance, mainly of riding comfort and running stability can be increased. Moreover, the number of assembling steps is decreased, allowing cost reduction.
In the structure described in U.S. Pat. No. 6,485,188, Japanese Laid-open Patent Publication No. 2001-1709 and Japanese Laid-open Patent Publication No. 2002-46409, while the cost can be reduced due to the reduction in the number of assembling steps, the fabrication of the outer ring raceway becomes troublesome and the manufacturing cost of the outer sing raceway is increased, so that it is difficult to reduce the cost sufficiently. The reason is as follows.
Firstly, in the conventional structure shown in FIG. 6, because of the heat applied to the outer ring 6a when casting the outer ring 6a in the knuckle 3a, the shape of the double row outer ring raceways 11c and 11d formed on the inner peripheral surface of the outer ring 6a is inevitably distorted. Therefore, after casting the outer ring 6a in the knuckle 3a, it is necessary to perform finishing process of the outer ring raceways 11c and 11d. This finishing process must be performed in a condition where the outer ring 6a which is integrated into the knuckle 3a, is set in the processing equipment for turning process. Since the knuckle 3a is much larger than the outer ring 6a and has a complex shape, the processing equipment becomes larger, and assembling of the knuckle 3a onto this processing equipment becomes troublesome. As a result, the cost for finishing process of the outer ring raceways 11c and 11d is increased and the cost reduction effect as a whole is not achieved.
As described in Japanese Laid-open Patent Publication No. 2001-1709 and Japanese Laid-open Patent Publication No. 2002-46409, the structure where the double row outer ring raceways are directly formed on the knuckle, or alternatively the outer ring is fixed to the knuckle by welding, causes problems similar to those of the conventional structure described in U.S. Pat. No. 6,485,188.